
Concept explainers
To review:
The time and the geological period when the last common ancestor of the seed plants as well as ferns lived.
Given:
The fFigure 1 shows the geological timescale of the origin of various embryophytes (land plants) including ferns and seed plants.
Figure 1: The geological timescale of the origin of various embryophytes.
Introduction:
Horsetails, ferns, and seed plants form a clade commonly referred as euphyllophytes. These are the true leaf plants. Another important characteristic feature of these plants is overtopping. It is a growth pattern, where one branch of the plant gets differentiated from the other and grows beyond that other branch. This feature gives an advantage to the plants in competition for light.

Explanation of Solution
It can be clearly observed from Figure 1 that the common ancestor of ferns as well as the seed plants originated about 390 million years ago in the mid-Devonian period of the Paleozoic era. Today, the ferns comprise of more than 12,000 species whereas there are about 15 known species of horsetails. The species of horsetails lie in a single genus, Equisetum. It can be noticed that horsetail species are more closely linked to ferns as compared to the seed plants.
Therefore, it can be concluded that common ancestor of ferns and seed plants arose in mid-Devonian about 390 million years ago.
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